Corrosion-resistant high-strength alloys, and method



Patented July 2, 1946 Nrr o TATE CORROSION-RESISTANT HIGH-STRENGTH ALLOYS, AND METHOD Paul P. Zeigler and Leland E. Householder, Louis ville, Ky., assignors to Reynolds Metals Company, Richmond, Va, a corporation of Delaware No Drawing. Application August 14, 1943, Serial No. 498,726

6 Claims. (01. 143-21.:0'

It has long been known that aluminum base 4 alloys of the class above described are susceptible to a form of corrosion known as stress cracking and many attempts have been made in the art to diminish stress cracking as, for example, by control of the proportion of copper and the addition of manganese and certain grain refining. metals such as titanium, boron, zirconium, molybdenum, cobalt, chromium, and vanadium. However, the problem of stress cracking of aluminum base alloys including those containing less than of zinc, has continued to such an ex tent that notwithstanding the high tensile, yield, and fatigue strengths of such alumlnumalloys aftersolution heat treatment and aging, their use has been materially restricted in certain important fields, as, for example, aluminum alloy sheet, forgings and extruded shapes.

It has been characteristic of these alloys that when they are under high stress, either external or internal in source, andare exposed to a cor-,

rosive condition or medium such as salt solutions, sea water, or other corrosive solutions, gases, etc., stress cracking occurs.

We have discovered that an aluminum base alloy containing magnesium, copper, and zinc in relatively high proportion, and containing at least one heavy metal of the class recognized as grain refiners, chromium and vanadium being examples, may be rendered so resistant to stress cracking by addition of small amounts of nickel, that an aluminum base alloy with a relatively high zinc content may be fully developed and may now have widespread use without material danger of stresscracking. Specifically, we provide for the first time an aluminum base magnesium-zinccopper alloy, solution heat treated and age hardened, which is satisfactory for aircraft construction and general engineering applications, be-

cause our alloy when exposed to corrosive conditions or mediums such as sea water, gases, etc., will be remarkably free from the peculiar cracking or structural failure known as stress cracking.

Our composition is basically:

Min. Max

Percent Percent Zn 4 5.0 10.0 Mg- 1.0 3. 5 011 0.5 3. 0 Ni 0.05 0.5 One ObIi more heavy metal grain refiners, prelirl' 0. 03 0. 5 Or 0. l0 0. 5 Aluminum and normal impurities Balance We also have discovered that solution heat treatment within a critical temperature range, 1. e., temperatures between760 F.-840 F., followed after preferably slow quenching by relatively high temperature-aging treatment with-1 in the temperature range 2'75,F.-350 F. substantially adds to the quality of the alloy with respect to resistance to stress cracking.

Although aging maybe conducted at a temperature somewhat below 300 F., as, for example, by prolonged aging at 275 R, we consider it preferable to age for shorter times'at the higher temperatures.

For a given temperature of quenching from solution heat treatment and for a given temperature of elevated temperature aging, relatively slow cooling by quenching in oil or hot or boiling water,

rather than by quenching in cold water, is desirab'ler The improvements in resistance to stress cracking secured by the above two primary methods of control are additive one to the other, and truly remarkable resistance to stress cracking can be secured by employing these above controls.

By employing our methods of solution heat treatment, quenching, and elevated temperature aging, we are able to obtain alloys with tensile strengths as high or higher than 80,000 pounds per square inch, with yield strengths as high or higher than 70,000 pounds per square inch, in combination with excellent resistance to stress cracking.

The time of the solution heat treatment will, as recognized in the art, vary with the mass of with the temperature employed. Generally eight hours or somewhat less will be sufilcient.

An example, of our basic composition is as follows:

After asolution heat treatment at various temperatures as listed below, sheet samples of the above compositions were quenched, and aged for eight hours at 320 F. Each sample was then stressed an amount equal to A of its yield strength and subjected-to corrosion in a 5.5% NaCl solution.

'lemp. of solution Time to show iniheat sample tial stress crack its 840 None in 85 days.

Our critical control of the primary factors of solution heat treatment and elevated temperature aging treatment will also to a substantial degree improve the resistance to stress cracking of aluminum base magnesium-zinc-copper alloys which do not have a nickel content but whichemploy the nickel and vanadium or chromium, or both, falling within our basic composition limits, provides an alloy of very substantial commercial value. I

With regard to the lower zinc content of our basic formula, an alloy containing the minimum zinc, magnesium and copper content, with nickel and vanadium or chromium, or both, as specified in the basic formula, would not possess the strength and hardness of our rescribed example of the basic formula. Nevertheless, the formula containing the minimum zinc content of 5% with 3.5% magnesium, 1 to 3% copper, and nickel and manganese with a view to reducing corrosion.

As an example, a composition embodying zinc 7.85%; magnesium 2.50%; copper 1.03%: manganese .81% and vanadium 07% was subjected to a solution heat treating temperature of 800 F. followed by normal water quenching and reheating to 320 F. with aging for 8 hours under that temperature. 'A sheet strip was stressed in an amount equal to of its yield strength and subjected to corrosion in a 5.5% NaCl solution: with the result that 69 days elapsed before appearance of the. first small surface crack, and

complete failure did not occur until after,89 days.

Zinc and magnesium are the principal hardening and strengthening components, although copper contributes to hardening and strengthen ing. In general, the higher the zinc content, the more susceptible to stress crackingare the particular compositions. which susceptibility to stress cracking is minimized by our use of nickel, the susceptibility to stress cracking being still further reduced by our prescribed solution heat treating, quenching and elevated temperature aging.

The example of our basic composition given above employs an intermediate proportion of zinc and magnesium with respect to the proportion range of the basic composition, and will illustrate a highly desirable formula, because the alloy obtained thereby will readily fabricate under usual commercial conditions, and, at the same time, the alloy possesses high strength and hardness. With an increase of the zinc content from 7.69% to 10% and corresponding increases of the ma nesium and copper constituents, the resulting alloy will be of great hardness, possessing certain dii'ficulties in fabrication. On the other hand, our formula with a maximum zinc content of 10% with the minimum magnesium content of 1%, the copper content ranging from 5% to 3%, with vanadium or chromium, or both, according to our basic formula, provides an alloy of high strength and hardness and of great value for commercial use.

Having described our invention, what we claim and desire to secure by Letters Patent. is as follows:

1. A method of heat-treating aluminum base alloys containing a relatively high proportion of zinc and not less than about 5%, and magnesium, copper, with or without other metals such as grain refining elements and hardeners, which consists in subjecting the alloy to solution heat treatment within the critical range of 760 F. 840 F., then quenching the alloy and then artificially aging the alloy within the critical temperature range of 275350 F.

2. 'A method of heat-treating aluminum base alloys containing a relatively high proportion of zinc and not less than about 5%, and magnesium, copper, with or without other metals such as grain refining elements and hardeners, which consists in subjecting the alloy to solution heat treatment within the critical range of 760 F.- 840 F.. slow quenching to room temperature, and then artificial aging at a temperature within the critical range of between 275 F.-350 F.

3. A wrought, heat-treated and artificially aged corrosion-resistant aluminum base alloy having a relatively high zinc content, composed of zinc 540%, magnesium l-3.5%, copper 0.5-3%, nickel .05-.5%, and at least one of the heavy metal grain refining elements consisting of vanadium .03-.5% and chromium .l0-.5%, balance aluminum, the alloy being produced by solution heat treatment within critical temperatures between 760 F.-840 F., quenching, and then artificial aging at temperatures between 275. F.-350 F.

4. A method of preparing and heat treating a wrought aluminum base alloy containing a relatively high proportion of zinc and from 1.0 to

3.5% magnesium and from 0.5 to 3% copper,

ing elements and hardeners, said method consisting of the step of adding at least one of the metals nickel and manganese to the alloy then subjecting the alloy to solution heat treatment within the temperature range of 760 to 840 F.. then the step of a gradual and continuous quenching from said solution heat treatment temperature down to room temperature, and then artificially aging the alloy within the critical temperature range of 275 to 350 F.

5. A method of preparing and heat treating wrought aluminum base alloys containing a relatively high proportion of zinc and from 1.0 to 3.5% magnesium and from 0.5 to 3% copper, with or without other metals such as grain refining elements and hardeners, said method consisting of the step of adding nickel to the alloy in the proportion of about ODS-0.5%, then sub- Jecting the alloy to solution heat treatment within the temperature range of 760 to 840 F., then the step of slowly quenching the alloy to room temperature, and then artificially aging the alloy within the critical temperature range of 275- 290 F.

6. A wrought, heat-treated artificially aged corrosion resistant aluminum base alloy having a relatively high zinc content, composed of zinc 540%, magnesium 13.5%, copper 05-13%, at least one of the metals nickel and manganese, 

